WA police aren't revealing if they were still in pursuit of a car when it collided head-on with another car carrying a mother and her two children, seriously injuring them.

The 37-year-old woman, her 11-year-old son and eight-year-old daughter were hospitalised after the sedan they were in crashed into a blue Commodore, earlier pursued by police, in East Victoria Park about 11.30pm (WST) on Monday.

The boy is in a critical condition and the girl is in a stable condition in Princess Margaret Hospital for Children.

A 21-year-old woman and two men in the Commodore were also hospitalised with serious injuries.

WA Assistant Police Commissioner Nick Staltari said police had attempted to stop the Commodore in Bentley, in Perth's southeast, before the crash but the car sped away, sparking a pursuit.

"The vehicle being pursued engaged in high speeds to get away from the police," Mr Staltari told reporters on Tuesday.

"The events of last night - the outcome is tragic.

"We would have much preferred that it had never occurred and our thoughts and sympathy are with the family involved.

"People who are being pursued or being called upon to stop need to stop. We can prevent these types of incidents if people stop."

Mr Staltari would not say if the pursuit had been aborted before the crash or if it was still active.

It is now the subject of an internal investigation.

Mr Staltari said police had an obligation to pursue people who would not stop their vehicles when told by police.

"Police have got an obligation to apply the law," Mr Staltari said.

"It's our duty to apprehend persons and unfortunately pursuits are part of that.

"If all people complied with the directions of police there would be no need for pursuits.

"Urgent duty driving is the subject of a strict policy. It's closely managed by a police operations inspector.

"This is being overseen by the internal affairs unit.

"They will critically examine the actions and behaviours of the officers of which the policy was applied."

It is understood the Commodore entered a roundabout the wrong way.